Download Lecture 4 - Hashim Ali

EC—331 Microprocessor and
Interfacing Techniques (3+1)
Lecture 4
Addressing Modes
Dr Hashim Ali
Spring — 2017
Department of Computer Science and Engineering
HITEC University Taxila
Addressing Modes in Microprocessor
(8086/88)
•
Addressing mode provides different ways for access an address to
given data to a processor.
•
When 8086 executes an instruction, it performs the specified
function on data. Operated data is stored in the memory location.
There are various techniques to specify address of data. These
techniques are called Addressing Modes.
•
Types of Addressing Modes:-
Data Addressing Modes
-
Program Memory Addressing Modes
-
Stack Memory Addressing Modes
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•
Data-Addressing Modes:
-
•
Program Memory Addressing Modes:
-
•
This mode is related to data transfer operation, that is, data is transferred
either from the memory to internal registers of 8086 processors or from one
register to another register.
‣ Example: MOV AX, DX
This mode involves program memory addresses during various operations.
‣ Example: JMP AX, in this instruction, the code execution control jumps
to the current code segment location addressed by the contents of AX
register.
Stack Memory Addressing Modes:
-
This mode involves stack registry operations.
‣ Example: PUSH AX, this instruction copies the contents of AX register
to the stack.
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Data-Addressing Modes
•
An addressing mode means the method by which an operand
can be specified in a register or a memory location.
•
8086/88 provide seven Addressing Modes:
1. Register Addressing
2. Immediate Addressing
3. Direct Data Addressing
4. Register Indirect Addressing
5. Base-Plus-Index Addressing
6. Register Relative Addressing
7. Base Relative-Plus-Index Addressing
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Opcode and Operand in Microprocessor
•
An opcode is a short of “operation code”
•
An opcode is a singe instruction can be executed by the CPU.
In machine language it is a binary or hexadecimal value such as B7 loaded into the
instruction register.
•
In assembly language mnemonic form an opcode is a command such as MOV or ADD
or JMP.
•
Example:
MOV AX, 1000H ; MOV is the opcode
; AX (register) is an operand
•
Operands are manipulated by the opcode. In this example, the operands are the register
AX and the value 1000H.
8-bit
Opcode
1 Byte
8-bit
8-bit
Operand
1 or 2 Byte
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Data-Addressing Modes
•
MOV instruction is a common and flexible instruction.
-
provides a basis for explanation of data-addressing
modes
•
Figure 3–1 illustrates the MOV instruction and defines the
direction of data flow.
•
Source is to the right and destination the left, next to the
opcode MOV.
-
an opcode, or operation code, tells the microprocessor
which operation to perform
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Figure 3–1 The MOV instruction showing the source,
destination, and direction of data flow.
•
The MOV AX, BX instruction transfers the contents of source register
(BX) into the destination register (AX).
-
The source never changes, but the destination always changes.
‣
-
MOV copies the source data into the destination.
The source and destination are often called operands.
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Figure 3–2 8086–Core2 data-addressing modes.
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1. Register Addressing
•
The most common form of data addressing.
-
•
Once register names learned, easiest to apply.
The microprocessor contains these 8-bit register names
used with register addressing: AH, AL, BH, BL, CH, CL,
DH, and DL.
-
16-bit register names: AX, BX, CX, DX, SP, BP, SI,
and DI.
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•
Register addressing transfers a copy of a byte or word
from the source register or contents of a memory location
to the destination register or memory location.
-
•
Ex: MOV CX, DX
Important for instructions to use registers that are the
same size.
-
Never mix an 8-bit with a 16-bit register, an 8- or a 16bit register with a 32-bit register
-
This is not allowed by the microprocessor and results
in an error when assembled.
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•
Use of registers to hold the data to be manipulated
•
Memory is not accessed when this addressing mode is
executed
•
Example:
MOV BX, DX ; copy the contents of DX into BX
MOV ES, AX ; copy the contents of AX into ES
ADD AL, BH ; add the contents of BH to contents of AL
•
Source and destination registers must have the same size
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2. Immediate Addressing
•
Transfers the source, an immediate byte or word of data,
into the destination register or memory location
•
The source operand is a constant
•
The operand comes immediately after the opcode
•
For this reason, this addressing mode executes quickly
•
Immediate addressing mode can be used to load
information into any of the registers except the segment
registers and flag registers.
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•
Example:
MOV AX, 2550H ; move 2550H into AX
MOV CX, 625
; load the decimal value 625 into CX
MOV BL, 40H
; load 40H into BL
•
The data must first be moved to a general-purpose
register and then to the segment register.
•
Example:
MOV AX, 2550H
MOV DS, AX
MOV DS, 0123H ; illegal instruction!
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•
In symbolic assembly language, the symbol # precedes
immediate data in some assemblers.
-
•
MOV AX,#1234H instruction is an example.
Most assemblers do not use the # symbol, but represent
immediate data as in the MOV AX,1234H instruction.
-
An older assembler used with some Hewlett-Packard
logic development does, as may others.
-
In this text, the # is not used for immediate data.
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•
The symbolic assembler portrays immediate data in many
ways.
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The letter H appends hexadecimal data.
•
If hexadecimal data begin with a letter, the assembler
requires the data start with a 0.
-
•
to represent a hexadecimal F2, 0F2H is used in assembly
language.
Decimal data are represented as is and require no special
codes or adjustments.
-
An example is the 100 decimal in the MOV AL,100
instruction.
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•
An ASCII-coded character or characters may be depicted
in the immediate form if the ASCII data are enclosed in
apostrophes.
-
•
Ex: MOV AL,’A’
Binary data are represented if the binary number is
followed by the letter B.
-
in some assemblers, the letter Y
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Ex: MOV CL,11001110B
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Parts of Assembly Language
•
Each statement in an assembly language program consists of four
parts or fields.
•
The leftmost field is called the label.
•
used to store a symbolic name for the memory location it represents
•
All labels must begin with a letter or one of the following special
characters: @, $, -, or ?.
•
a label may any length from 1 to 35 characters
•
The label appears in a program to identify the name of a memory
location for storing data and for other purposes.
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•
•
•
The next field to the right is the opcode field.
-
designed to hold the instruction, or opcode
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the MOV part of the move data instruction is an example of an
opcode
Right of the opcode field is the operand field.
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contains information used by the opcode
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the MOV AL,BL instruction has the opcode MOV and operands
AL and BL
The comment field, the final field, contains a comment about the
instruction(s).
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comments always begin with a semicolon (;)
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3. Direct Data Addressing
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Moves a byte or word between a memory location and a register.
•
The data is in some memory location(s) and the address of the
data in memory comes immediately after the instruction
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usually a 3-byte long instruction
•
This address is the offset address
•
Example:
MOV AL, [8088]
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; move content of DS:8088 into AL
The physical address is calculated by combining the contents of
offset location 2400 with DS
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•
Example:
Find the physical address of the memory location and its contents after the
execution of the following, assuming that DS = 1512H.
MOV AL, 3BH
MOV [3518], AL
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Solution:
-
First 3BH is copied into AL
-
Then in line two, the contents of AL are moved to logical address DS:
3518 which is 1512:3518.
-
Shifting DS left and adding it to the offset gives the physical address of
18638H (15120H + 3518H = 18638H).
-
After the execution of the second instruction, the memory location with
address 18638H will contain the value 3BH.
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Few Examples
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4. Register Indirect Addressing
•
Transfers a byte or word between a register and a memory
location addressed by an index or base register.
•
The address of the memory location where the operand
resides is held by a register.
•
The registers used for this purpose are SI, DI, and BX.
•
They must be combined with DS (by default) in order to
generate the 20-bit physical address.
-
The register BP uses the stack segment (SS) by default.
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•
•
For example:
-
MOV AL, [BX]
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MOV AL, [BP]
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MOV AL, [SI]
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MOV AL, [DI]
Segment override prefix symbols can
be used if you wish to access data in
different segments.
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MOV AL, CS:[BX]
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MOV AL, DS:[BP]
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MOV AL, SS:[SI]
-
MOV AL, ES:[DI]
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•
Example:
MOV AX, [BX] ; moves into AX the contents of the memory
location pointed to by DS:BX, 1000:1234
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The physical address is calculated as
1000x10+1234=11234H
•
The same rules apply when using register SI or DI.
•
Example:
MOV CL, [SI]
; move contents of DS:SI into CL
MOV [DI], AH
; move contents of AH into DS:DI
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•
Example:
-
•
Assume that DS = 1120H, SI = 2498H, and AX = 17FEH Show
the contents of memory locations after the execution of
MOV [SI], AX ; move contents of AX into DS:SI
Solution:
-
The contents of AX are moved into memory locations with logical
address DS:SI and DS:SI + 1 (Two bytes because of AX)
-
The physical address starts at DS (shifted left) + SI = 13698.
According to the little endian convention,
-
Low address 13698H contains FE, the low byte,
-
High address 13699H will contain 17, the high byte.
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Few Examples
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5. Base-Plus-Index Addressing
•
Transfers a byte or word between a register and the
memory location addressed by a base register (BP or
BX) plus an index register (DI or SI).
•
Combining based and indexed addressing modes.
•
One base register and one index register are used.
•
Examples:
MOV [BX+DI], CL ; move contents of CL into DS:BX+DI
-
Physical Address = DSx10 + BX+DI
MOV CH, [BX+SI] ; move contents of the DS:BX+SI into CH
-
Physical Address = DSx10 + BX+SI
MOV AH, [BP+DI] ; move contents of the SS:BP+SI into AH
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Physical Address = SSx10 + BP+DI
MOV [BP+SI], AL ; move contents of AL into SS:BP+SI
-
Physical Address = SSx10 + BP+SI
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Few Examples
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6. Register Relative Addressing
•
Moves a byte or word between a register and the
memory location addressed by an index or base register
plus a displacement.
•
The data in a segment of memory are addressed by
adding the displacement to the contents of a base or an
index register (BP, BX, DI, or SI).
•
Examples:
MOV AX, [BX+4]
-
Physical Address = DSx10 + BX+4
MOV CH, [SI+5]
-
; move contents of the DS:DI+1 into AH
Physical Address = DSx10 + DI+1
MOV [BP+2], AL
-
; move contents of the DS:SI+5 into CH
Physical Address = DSx10 +SI+5
MOV AH, [DI+1]
-
; move contents of DS:BX+4 into AX
; move contents of AL into SS:BP+2
Physical Address = SSx10 + BP+2
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•
Example:
•
Assume that DS = 4500, SS = 2000, BX = 2100, SI = 1486, DI = 8500,
BP= 7814, and AX = 2512. Show the exact physical memory location
where AX is stored in each of the following. All values are in hex.
1- MOV [BX+20], AX
2- MOV [SI+10], AX
3- MOV [DI+4], AX
4- MOV [BP+12], AX
•
Solution:
Physical Address = segment reg. x 10 + (offset reg.) + displacement
1- DS:BX+20
2- DS:SI+10
3- DS:DI+4
4- SS:BP+12
location 47120 = (12) and 47121 = (25)
location 46496 = (12) and 46497 = (25 )
location 4D504 = (12) and 4D505 = (25)
location 27826 = (12) and 27827 = (25)
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Few Examples
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7. Base Relative-Plus-Index Addressing
•
The base relative-plus-index addressing mode is similar to the
base-plus-index addressing mode, but adds a displacement
besides using a base register and an index register to form the
memory address.
•
This type of addressing mode
often addresses a two-dimensional
array of memory data.
•
The data in a segment of memory are addressed by adding the
displacement to the contents of a base and an index register (BP, BX,
DI, or SI).
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•
Examples:
MOV [BX+DI+1], AX
-
; move contents of AX into
DS:BX+DI+1
Physical Address = DSx10 + BX+DI+1H
MOV AX, [BX+SI+10]
-
; move contents of the
DS:BX+SI+10 into AX
Physical Address = DSx10 + BX+SI+10H
MOV AH, [BP+DI+3]
-
-
; move contents of the SS:BP+SI+3
into AH
Physical Address = SSx10 + BP+DI+3H
MOV AX, FILE[BX+DI] ; move contents of the
DS:FILE+BX+DI into AX
Physical Address = DSx10 + BX+DI+FILE
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Few Examples
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Offset Registers For Various Segments
•
The following Table provides a summary of the offset
registers that can be used with the four segment registers
of the 8086/8088.
Segment Register
CS
DS
ES
SS
Offset Register
IP
SI, DI, BX
SI, DI, BX
SP, BP
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Easy Way to Remember 8086/88
Addressing Modes
•
Total 17 different memory addressing modes:-
DISP, [BX], [BP], [SI], [DI], [BX+DISP], [BP+DISP],
[SI+DISP], [DI+DISP], [BX+SI], [BX+DI], [BP+SI],
[BP+DI],[BX+SI+DISP], [BX+DI+DISP],
[BP+SI+DISP], and [BP+DI+DISP]
‣
DISP = Displacement
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Assembly Language
•
There is a one-to-one relationship between assembly and machine
language instructions.
•
What is found is that a compiled machine code implementation of a
program written in a high-level language results in inefficient code.
-
•
More machine language instructions than an assembled version
of an equivalent handwritten assembly language program.
Two key benefits of assembly language programming.
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It takes up less memory
-
It executes much faster
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Languages in-terms of Applications
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One of the most beneficial uses of assembly language programming is real-time applications.
•
Real time means the task required by the application must be completed before any other input to
the program that will alter its operation can occur
•
For example the device service routine which controls the operation of the floppy disk drive is a
good example that is usually written in assembly language
•
Assembly language not only good for controlling hardware devices but also performing pure
software operations
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searching through a large table of data for a special string of characters
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Code translation from ASCII to EBCDIC
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Table sort routines
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Mathematical routines
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Assembly language: perform real-time operations
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High-level languages: Those operations mostly not critical in time.
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Assembly Instructions
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Assembler Directives
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Model: Selects the size of the memory model.
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.MODEL SMALL : Reserves 64KB memory for Code Segment (CS) and 64KB for
Data Segment (DS).
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.MODEL MEDIUM : CS can exceed 64KB but DS must fit in 64KB of memory.
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.MODEL COMPACT : CS is of fixed 64KB size but DS can vary.
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.MODEL LARGE : Both CS & DS can exceed 64KB but only one of them.
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.MODEL HUGE : Both can exceed its 64KB memory size.
-
.MODEL TINY : Used with compact (com) files in which CS & DS must fit into
64KB.
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Assembly Code Template
.MODEL SMALL
.STACK
; beginning of the stack segment
.DATA
; beginning of the data segment
.CODE
; beginning of the code segment
Ex: .DATA
DATAW
DATAB
SUM
DW
DB
DB
213FH
52H
?
;nothing stored but storage is assigned
Ex: .CODE
ProcedureName PROC Attribute
…
ProcedureName ENDP
END ProcedureName
; name of the procedure
; procedure statements
PROC is a statement used to indicate the beginning of a procedure or subroutine.
ENDP indicates the end of the procedure.
ProcedureName may be any valid identifier.
Attribute is NEAR if the Procedure is in the same code segment as the calling program; or FAR if in a different code
segment.
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Sample Program
; This program displays “Hello, World!”
.MODEL SMALL
.STACK 100H
.DATA
message DB “Hello, World!”, 0dh, 0ah, ‘$’ ; newline+eoc
.CODE
MAIN PROC
mov ax, @data ; address of DS
mov ds, ax
mov ah, 9
mov dx, offset message ; displacement message starting at 0
int 21h
mov ax, 4C00h ; halt the program and return
int 21h
main endp
end main
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PROGRAM MEMORY — ADDRESSING
MODES
•
Program memory-addressing modes, used with the JMP
and CALL instructions, consist of three distinct forms:
1. Direct Program Memory Addressing
2. Relative Program Memory Addressing
3. Indirect Program Memory Addressing
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1. Direct Program Memory Addressing
•
Used for all jumps and calls by early microprocessor;
also used in high-level languages, such as BASIC.
-
GOTO and GOSUB instructions
•
The microprocessor uses this form, but not as often as
relative and indirect program memory addressing.
•
The instructions for direct program memory addressing
store the address with the opcode.
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Figure 3–14 The 5-byte machine language version of a JMP
[10000H] instruction.
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•
This JMP instruction loads CS with 1000H and IP with
0000H to jump to memory location 10000H for the next
instruction.
-
•
an intersegment jump is a jump to any memory
location within the entire memory system
Often called a far jump because it can jump to any
memory location for the next instruction.
-
in real mode, any location within the first 1M byte
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•
The only other instruction using direct program
addressing is the intersegment or far CALL instruction.
•
Usually, the name of a memory address, called a label,
refers to the location that is called or jumped to instead of
the actual numeric address.
•
When using a label with the CALL or JMP instruction,
most assemblers select the best form of program
addressing.
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2. Relative Program Memory
Addressing
•
Not available in all early microprocessors, but it is
available to this family of microprocessors.
•
The term relative means “relative to the instruction
pointer (IP)”.
•
The JMP instruction is a 1-byte instruction, with a 1-byte
or a 2-byte displacement that adds to the instruction
pointer.
•
An example is shown in Figure 3–15.
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Figure 3–15 A JMP [2] instruction. This instruction skips
over the 2 bytes of memory that follow the JMP instruction.
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3. Indirect Program Memory
Addressing
•
The microprocessor allows several forms of program
indirect memory addressing for the JMP and CALL
instructions.
•
In 80386 and above, an extended register can be used to
hold the address or indirect address of a relative JMP or
CALL.
-
for example, the JMP EAX jumps to the location
address by register EAX
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•
If a relative register holds the address, the jump is
considered to be an indirect jump.
•
For example, JMP [BX] refers to the memory location
within the data segment at the offset address contained in
BX.
•
at this offset address is a 16-bit number used as the offset
address in the intrasegment jump
•
this type of jump is sometimes called an indirect-indirect
or double-indirect jump
•
Figure 3–16 shows a jump table that is stored, beginning
at memory location TABLE.
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Figure 3–16 A jump table that stores addresses of various
programs. The exact address chosen from the TABLE is
determined by an index stored with the jump instruction.
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3–3 STACK MEMORY-ADDRESSING
MODES
•
The stack plays an important role in all microprocessors.
-
•
holds data temporarily and stores return addresses used
by procedures
Stack memory is LIFO (last-in, first-out) memory
-
describes the way data are stored and removed from
the stack
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•
Data are placed on the stack with a PUSH instruction;
removed with a POP instruction.
•
Stack memory is maintained by two registers:
•
-
the stack pointer (SP or ESP)
-
the stack segment register (SS)
Whenever a word of data is pushed onto the stack, the
high-order 8 bits are placed in the location addressed by
SP – 1.
-
low-order 8 bits are placed in the location addressed by
SP – 2
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•
The SP is decremented by 2 so the next word is stored in
the next available stack location.
-
the SP/ESP register always points to an area of memory
located within the stack segment.
•
In protected mode operation, the SS register holds a
selector that accesses a descriptor for the base address of
the stack segment.
•
When data are popped from the stack, the low-order 8 bits
are removed from the location addressed by SP.
-
High-order 8 bits are removed; the SP register is
incremented by 2.
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Figure 3–17 The PUSH and POP instructions: (a) PUSH BX
places the contents of BX onto the stack; (b) POP CX
removes data from the stack and places them into CX. Both
instructions are shown after execution.
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•
Note that PUSH and POP store or retrieve words of data
—never bytes—in 8086 - 80286.
•
80386 and above allow words or doublewords to be
transferred to and from the stack.
•
Data may be pushed onto the stack from any 16-bit
register or segment register.
-
•
in 80386 and above, from any 32-bit extended register
Data may be popped off the stack into any register or any
segment register except CS.
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•
PUSHA and POPA instructions push or pop all except
segment registers, on the stack.
•
Not available on early 8086/8088 processors.
•
80386 and above allow extended registers to be pushed
or popped.
-
64-bit mode for Pentium and Core2 does not contain a
PUSHA or POPA instruction
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